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Fluorescence

In this series, Colin Baker of Bedford School provides spectacular demonstrations, designed to capture the student's imagination. The demonstrations are easy to prepare, safe to dispose of and they work. In this issue: fluorescence

Nothing tends to imprint chemical facts upon the mind so much as the exhibition of interesting experiments - Samuel Parkes, 1816

Source: Colin Baker

Energy takes many forms, one of which is light. Light can be created when energy is supplied to an appropriate material. There are two common ways for this to occur: incandescence and luminescence.

Light fantastic

Incandescence (from the Latin incandescere meaning glow, which is derived from the root candere meaning to be white or glisten) is when a substance emits light as a result of being heated to a high temperature - ie the substance begins to glow. Incandescence occurs when a metal in a flame begins to glow 'red hot'. A light bulb will glow 'white hot' and the Sun and stars glow by incandescence.

Luminescence (from the Latin lumen, meaning light) is 'cold light', ie light emitted by a substance which is caused by any other reason than a large rise in its temperature. In luminescence, an energy source other than heat (a photon in photoluminescence; an electron in electroluminescence) promotes an electron of an atom from its lowest energy (ground) state into an higher energy (excited) state. When the electron returns to its ground state, it gives back the energy in the form of light. A quantum of energy forms a photon of light. Depending on how long the excited state takes to relax, defines two types of luminescence: if it happens very quickly, typically within 10 ns, this is fluorescence; if it slower, over a period of seconds, this is phosphorescence. These two types of luminescence are identified by the type of glow observed after the excitation source is removed: a very short-lived glow (fluorescence); or one that persists (phosphorescence).

This demonstration highlights a few fluorescent dyes, which have important applications in microscopy, medicine, forensic science and lasers, and shows that some everyday substances contain fluorescent chemicals too.

Source: Colin Baker

Kit

An ultraviolet light source;

Fluorescein (solid disodium salt);

Eosin;

Coumarin;

Daz washing powder;

Schweppes tonic water;

Brown egg shells.

Procedure

If this demonstration is to be successful, you will need to have a word with your technical staff to see whether they can make an ultraviolet light source. Our box is made from mdf and a uv strip light borrowed from the physics department.

Prepare the following solutions:

fluorescein - half a spatula measure in one litre of distilled water;

eosin - five drops of liquid in one litre of distilled water;

coumarin - half a spatula measure in one litre of distilled water;

Daz washing powder - five heaped spatula measures in one litre of water. The powder does not dissolve very well, so decant off and use the top solution;

Schweppes tonic water - one litre bottle;

brown egg shells - crush into small pieces, dissolve in concentrated hydrochloric acid, decant off top solution and dilute to one litre.

Place each of the solutions in the uv light box and observe fluorescence.

Safety

Eosin may be harmful; fluorescein and coumarin may act as irritants and concentrated hydrochloric acid is extremely corrosive. Coumarin is toxic if swallowed.

Special tips

You may have to alter the recipe a little to give the required colour intensity but this is simple trial and error. You may have to persevere with the chicken eggs because, depending on the chickens' diet, the egg shells may not fluoresce. However, if you get a batch of suitable free range eggs, then the colour is a beautiful red.

Teaching goals

There are many examples of everyday materials that contain fluorescent chemicals, which can provide a chemical signature to aid identification. White paper made before 1950 does not contain fluorescent chemicals. Optical brighteners, eg sulfonic stilbene (trans -1,2-diphenylethene) derivatives, were added to paper after this date to make it appear brighter. By placing white paper under uv light, forgery of historical documents could be detected. Paper used for today's bank notes does not contain optical brighteners so forged notes can be detected by checking for fluorescence.

Scene of crime officers use uv light to aid forensic analysis because body fluids (blood, urine and semen) all fluoresce. Antifreeze has fluorescent additives that are used by forensic scientists to reconstruct car accident scenes.

Chlorophyll makes plants green, but the pigment fluoresces a blood red colour. Some tooth pastes contain compounds that glow blue to keep teeth from appearing yellow, and stamps are printed with inks that contain fluorescent dyes. Quinine is added to tonic water to produce a bitter flavour and under uv light glows blue-white.

Anyone who has been to a disco will have noticed that certain fabrics fluoresce under uv light. However, it is not the fabric that glows but the stilbene-based optical brighteners which are present in detergents and remain on clothes after washing and rinsing.

Although not included in this demonstration (because they are best seen on a small scale), solutions of vitamin A and the B vitamins thiamine, niacin, and riboflavin are strongly fluorescent. If you crush a vitamin B12 tablet and dissolve it in vinegar, the solution will glow bright yellow under uv light.

A search of the Internet yields several useful definitions for terms relating to luminescence. For example, the luminescence section of the Fluorescent Mineral Society website warns that, 'Photoluminescence should not be confused with reflection, refraction, or scattering of light, which cause most of the colours you see in daylight or bright artificial lighting. Photoluminescence is distinguished in that the light is absorbed for a significant time, and generally produces light of a frequency that is lower than, but otherwise independent of, the frequency of the absorbed light'.

As well as offering video clips of many illuminating demonstrations, the University of Leeds' Delights of chemistry website has an online appendix with concise definitions of luminescence, fluorescence and phosphorescence.